The present invention relates to a method and apparatus for recording and playing back PCM signals from a magnetic tape and more particularly to a synchronization protecting apparatus well suited for recording and playing back a PCM sound when the overlap area of an 8 mm video tape recorder is expanded.
In the past, the track format 31 of the conventional 8 mm video tape recorder has been determined as shown in FIG. 12. As shown in the Figure, a video signal 34 is recorded on an area 31 V corresponding to the cylinder head wrapping portion of 185 degrees and a PCM sound signal 37 time-base compressed to sterephonic sound data for one field is recorded on an area 31P overlapping 36-degrees in the direction of the head scan entering side of the video signal 34. The 36-degree overlap area 31P includes a scan starting section 39 (a margin section for a head scan starting point), a preamble 38 (a clock regenerating signal for clock pull-in purposes), the PCM sound signal 37, a postamble 36 (a margin section during the period of after recording) and a VP guard 35 (a guard section between the video signal and the PCM sound signal), and the PCM sound data 37 begins at the position of 5 degrees from the head scan entering side. Then, the signals 38 to 36 are subjected to biphase mark modulation and recorded on the magnetic tape. With this PCM sound of the conventional 8 mm video tape recorder, the sampling frequency is 31.5 kHz and the number of quantization bits is 10. These values are inferior in sound quality as compared with the sampling frequencies of 48 kHz and 44.1 kHz and the number of quantization bits or 16 bits which are the main trends in the field of CD and DAT. However, since the PCM sound whose sampling frequency is 48 kHz and whose number of quantization bits is 16 has an amount of information which is about three times that of the conventional PCM sound, any attempt to realize such amount of information in the same overlap area 31P of 36 degrees as previously by utilizing the conventional PCM sound system requires a line recording density of about 3 times the conventional one.
Thus, the optimization of the correcting codes has been effected to reduce the line recording density and yet the line recording density of about 2.5 times has been required. And, in an attempt to realize a high-density magnetic recording, a high-performance tape has been used to optimize the modulation system and yet it has been limited to the recording and playback of the line recording density of about 2 times that of the conventional PCM sound.
Therefore, in order that a PCM sound whose sampling frequency is 48 kHz and number of quantization bits is 16 may be realized in an 8 mm video tape recorder, there is no alternative but to expand the overlap area 31P of 36 degrees. FIG. 12 shows a new 8 mm track format 32 in which the overlap area is expanded by 5 degrees. By so expanding the overlap area 32P into a linear audio track which is not used, it is possible to realize a PCM sound having the sampling frequency of 48 kHz and the number of quantization bits of 16 with a line recording density of slightly over 2 times the conventional one (see JP-A-1-119966).
By expanding the overlap area by 5 degrees, it is possible to realize a sound having a sampling frequency of 48 kHz and 16 quantization bits in terms of line recording density. However, as shown in FIG. 12, expanding the overlap area by 5 degrees reduces the distance from the lower edge of a magnetic tape 10 and hence the head playback output at around the head entry side of the track 32. This is due to the fact that upon the entry of the cylinder head, the lower edge of the magnetic tape 10 is turned up thereby increasing the gap between the head and the tape. In this way, at around the head entry side the S/N ratio is deteriorated by the reduced playback output and hence the error rate is deteriorated.
On the other hand, the vicinity of the head entry side is also near to the head of PCM sound data 42 and therefore a burst error tends to occur. This is due to a synchronization error. The PCM sound data 42 has a format 50 such as shown in FIG. 13 and the sound data for one field is divided into several blocks. Each block is added, as a header 51, with a synchronizing signal, ID code (control signal), block address and parity code as shown in FIG. 3 and they serve important roles such as the synchronization for converting serial signal data into-parallel signal data in terms of symbols and the generation of the accurate RAM address for the sound data in the block. As a result, a measure is taken so that during the period of playback the synchronizing signal and the block address in the header 51 are protected by referring to the information preceding several blocks thereby reducing the effect due to any desynchronization or block address error. However, there is no information to be referred to for the synchronizing signal and the block address in the leading block and the protection is deteriorated. More specifically, if an error is caused in the header 51 of the leading block, during the data conversion in terms of symbols a synchronization error or RAM address error is caused thereby causing a situation equivalent to the occurrence of a burst error of the block length even if the sound data in the block is correct entirely.
In this manner, the expansion of the overlap by 5 degrees deteriorates the playback output at around the head of the track as well as the error rate. Also, the probability of a synchronization error or address error in the leading block is increased and a burst error of the block length tends to occur. As a result, the probability of generating sound data by interpolation is increased thus giving rise to a problem of deterioration in the sound quality.
Disclosed in JP-A-60-247867 is a technique for recording a synchronizing signal pattern in the area of synchronizing clock signals for signal playback purposes with a view to preventing such synchronization error in the leading block of a track.